* Moves introduction to README so it shows on the index page.
* Made intro text more generic to account for object doubles.
* Rename old introduction feature to "instance doubles" for symmetry
with class and object double features.

RSpec::Mocks::ExampleMethods is mixed into a context
where users freely define their own methods. As such,
it's best to not define private helper methods as the
user may inadvertently overwrite them. These methods
were all stateless, anyway, so they work equally well
as class methods and this creates less surface area
for override problems.